Implementation of state rollback in /sync

Implementation of SPEC-254: roll back the state dictionary to how it looked at
the start of the timeline.

Merged PR https://github.com/matrix-org/synapse/pull/373

synapse/rest/client/v2_alpha/sync.py

@@ -20,6 +20,7 @@ from synapse.http.servlet import (
 )
 from synapse.handlers.sync import SyncConfig
 from synapse.types import StreamToken
+from synapse.events import FrozenEvent
 from synapse.events.utils import (
     serialize_event, format_event_for_client_v2_without_event_id,
 )
@@ -256,7 +257,13 @@ class SyncRestServlet(RestServlet):
         :rtype: dict[str, object]
         """
         event_map = {}
-        state_events = filter.filter_room_state(room.state.values())
+        state_dict = room.state
+        timeline_events = filter.filter_room_timeline(room.timeline.events)
+
+        state_dict = SyncRestServlet._rollback_state_for_timeline(
+            state_dict, timeline_events)
+
+        state_events = filter.filter_room_state(state_dict.values())
         state_event_ids = []
         for event in state_events:
             # TODO(mjark): Respect formatting requirements in the filter.
@@ -266,7 +273,6 @@ class SyncRestServlet(RestServlet):
             )
             state_event_ids.append(event.event_id)
 
-        timeline_events = filter.filter_room_timeline(room.timeline.events)
         timeline_event_ids = []
         for event in timeline_events:
             # TODO(mjark): Respect formatting requirements in the filter.
@@ -297,6 +303,63 @@ class SyncRestServlet(RestServlet):
 
         return result
 
+    @staticmethod
+    def _rollback_state_for_timeline(state, timeline):
+        """
+        Wind the state dictionary backwards, so that it represents the
+        state at the start of the timeline, rather than at the end.
+
+        :param dict[(str, str), synapse.events.EventBase] state: the
+            state dictionary. Will be updated to the state before the timeline.
+        :param list[synapse.events.EventBase] timeline: the event timeline
+        :return: updated state dictionary
+        """
+        logger.debug("Processing state dict %r; timeline %r", state,
+                     [e.get_dict() for e in timeline])
+
+        result = state.copy()
+
+        for timeline_event in reversed(timeline):
+            if not timeline_event.is_state():
+                continue
+
+            event_key = (timeline_event.type, timeline_event.state_key)
+
+            logger.debug("Considering %s for removal", event_key)
+
+            state_event = result.get(event_key)
+            if (state_event is None or
+                    state_event.event_id != timeline_event.event_id):
+                # the event in the timeline isn't present in the state
+                # dictionary.
+                #
+                # the most likely cause for this is that there was a fork in
+                # the event graph, and the state is no longer valid. Really,
+                # the event shouldn't be in the timeline. We're going to ignore
+                # it for now, however.
+                logger.warn("Found state event %r in timeline which doesn't "
+                            "match state dictionary", timeline_event)
+                continue
+
+            prev_event_id = timeline_event.unsigned.get("replaces_state", None)
+            logger.debug("Replacing %s with %s in state dict",
+                         timeline_event.event_id, prev_event_id)
+
+            if prev_event_id is None:
+                del result[event_key]
+            else:
+                result[event_key] = FrozenEvent({
+                    "type": timeline_event.type,
+                    "state_key": timeline_event.state_key,
+                    "content": timeline_event.unsigned['prev_content'],
+                    "sender": timeline_event.unsigned['prev_sender'],
+                    "event_id": prev_event_id,
+                    "room_id": timeline_event.room_id,
+                })
+            logger.debug("New value: %r", result.get(event_key))
+
+        return result
+
 
 def register_servlets(hs, http_server):
     SyncRestServlet(hs).register(http_server)

synapse/storage/events.py

@@ -831,7 +831,8 @@ class EventsStore(SQLBaseStore):
                 allow_none=True,
             )
             if prev:
-                ev.unsigned["prev_content"] = prev.get_dict()["content"]
+                ev.unsigned["prev_content"] = prev.content
+                ev.unsigned["prev_sender"] = prev.sender
 
         self._get_event_cache.prefill(
             (ev.event_id, check_redacted, get_prev_content), ev
@@ -888,7 +889,8 @@ class EventsStore(SQLBaseStore):
                 get_prev_content=False,
             )
             if prev:
-                ev.unsigned["prev_content"] = prev.get_dict()["content"]
+                ev.unsigned["prev_content"] = prev.content
+                ev.unsigned["prev_sender"] = prev.sender
 
         self._get_event_cache.prefill(
             (ev.event_id, check_redacted, get_prev_content), ev